1. Field of the Invention
The present invention relates to a reinforced mica paper and also a method of manufacturing the paper, and more particularly, to a sheet of reinforced mica paper or a tape of reinforced mica paper tape for use in a fire-resistant electric wire, and also a method of manufacturing the sheet or tape.
2. Description of the Related Art
A fire-resistant electric wire normally consists of a single-core conductor or multi-core conductor. In the fire-resistant electric wire, a reinforced mica paper sheet or a reinforced mica paper tape is used as a fire-resistant insulating folium. The Fire Defence Agency Notification (Standards) in Japan requests the following strict characteristics for the fire-resistant electric wire. That is, (1) the fire-resistant electric wire must withstand a temperature of 840.degree. C. (for 30 minutes) under prescribed load and electric charge conditions, (2) the fire-resistant electric wire must have an insulation resistance of 0.4 M.OMEGA. or more (at 840.degree. C.), (3) the fire-resistant electric wire must pass a dielectric strength test of AC 1,500 V, and the like. In standards in foreign countries, e.g., International Standards IEC331, in addition to a regulation such that the fire-resistant electric wire must withstand a temperature of 750.degree. C. for three hours, strict regulations ranging from 750.degree. C. to 900.degree. C. are provided to meet situations in the respective countries. The reinforced mica paper sheet or the like, therefore, has an important role as an insulting film.
For example, the reinforced mica paper tape normally consists of reinforced mica paper having a thickness of 0.09 to 0.11 mm (120 g/m.sup.2 to 180 g/m.sup.2) as a base material. A glass fiber fabric (or a non-woven fabric) having a thickness of about 0.03 mm is formed as a reinforcing material layer on the base material by using an adhesive consisting of a condensation or addition-polymerization type pressure-sensitive silicone resin paint (varnish).
The reinforced mica paper tape is wound around a conductor by a high-speed winding machine. This insulation normally has a winding thickness of about two 1/2-lap turns ((0.15 mm.times.2).times.2=0.6 mm).
The tape of reinforced mica paper, however, has no satisfactory dielectric strength great enough to satisfy wide-range fire resistances in foreign countries. In order to increase the dielectric strength, the thickness of the tape or an apparent density of the reinforced mica paper need only be increased. In the former case, however, the thickness of an electric cable is increased to degrade a space factor. In the latter case, since flexibility of a base material is lost, no satisfactory adhesion properties can be obtained when the tape is wound around a conductor. Therefore, as in the former case, a space factor is degraded. A glass fiber fabric as a reinforcing material is stable around a temperature of 700.degree. C. In a high-temperature atmosphere (700.degree. C. to 900.degree. C.), however, since insulation characteristics of the glass fiber fabric are acceleratedly degraded, the glass fiber fabric can no longer serve as a reinforcing material. Therefore, the gas generated within the paper when the paper is heated directly passes through pores in reinforced mica paper to rapidly decrease the insulation resistance of the paper.
Conventional examples are disclosed in Published Unexamined Japanese Utility Model Application No. 56-170698 (former) and Published Examined Japanese Utility Model Application No. 64-1710 (latter). The former example is an electric insulating mica tape in which a backing material is improved to improve mechanical strength and resin impregnation properties. The latter example is a fire-resistant electric cable insulating tape in which the thickness and the number of each of wefts and warps of glass fibers constituting a woven or non-woven fabric are improved.